KR101045249B1 - Plasma processing method - Google Patents

Abstract

An object of the present invention is to avoid damage of the nozzle by the spatter generated at the time of piercing, to suppress the deterioration of the nozzle by the pilot arc, and to significantly extend the life of the nozzle,

For this purpose, the initial height, which is the distance between the plasma torch and the steel sheet when generating the plasma arc and the piercing is started, is positioned at the same height as the cutting height, which is the distance between the plasma torch and the steel sheet when cutting, and the plasma arc Immediately after generation, the plasma torch is raised to the pierce height, which is a position away from the steel plate W, rather than the initial height, and pierced at the pierce height position. After completion of the piercing, the plasma torch is lowered to the cutting height to start cutting. . In addition, the pilot current is interrupted by the off action of the transistor immediately after the transition from the pilot arc to the main arc.

Description

Plasma processing method {PLASMA WORKING METHOD}

1 is an overall perspective view of a plasma cutting device according to an embodiment of the present invention.

Fig. 2 (a) is a schematic structural diagram of a main part of the plasma cutting device in the present embodiment, and Figs. 2 (b) to (f) are diagrams for explaining the plasma arc generating circuit and its operation.

3 is an operation explanatory diagram of the piercing of the present embodiment.

FIG. 4 (a) is a correlation diagram of pilot current-initial height, and FIG. 4 (b) is a correlation diagram of nozzle life frequency-pilot current.

5 is an operation explanatory diagram of a conventional piercing.

(Explanation of symbols for the main parts of the drawing)

One … Plasma cutting device 2. Cutting plate

3…. Frame 4... Running beam

5…. Carriage 6... Plasma torch

7. X-axis motor 8. X axis rail

9... Y-axis motor 10... Y-axis rail

11. Z-axis motor 12.. Torch cable

13. Relay box 14... Power cable

15... Plasma power supply unit 15a... Constant current power supply                 

16. Base material cable 17. electrode

18. Nozzle 18a... Nozzle orifice

20... Power line 21. Pilot current circuit

22. Main current circuit 23. High frequency generator

24. High Frequency Bypass Capacitor

25…. Resistance 26. Switching transistor

27. Pilot current detector 28. Main current detector

51. Plasma torch CH… Cutting height

FH… Initial height Im… Main current

Ip… Pilot current MA... Main arc

PA… Pilot arc PH... Pierce Height

R… Working gas passage W… Grater

The present invention relates to a plasma processing method for piercing or cutting by a plasma arc generated from a plasma torch, and more particularly, to a plasma processing method including a piercing operation for improving the nozzle life of a plasma torch. .

Plasma processing using the high-density heat of the arc having the plasma pillars mechanically and electrically collected has been widely used for practical use because it can achieve high precision and high efficiency cutting. The plasma processing method is generally performed by generating a plasma arc from the plasma torch, cutting the plasma arc by stably maintaining the plasma arc, and moving the plasma torch.

The plasma torch includes an electrode and a nozzle disposed to cover the electrode, and is configured to eject plasma gas toward the cutting material through a passage partitioned between the electrode and the nozzle. When cutting the cutting material using the plasma torch, first, a pilot arc is generated between the electrode and the nozzle, and then the pilot arc is grown so as to transfer to the main arc, thereby converting the plasma arc of high temperature and high density energy into the electrode and the blood. Occurs between the cuttings.

In the case where the steel sheet is cut into various shapes using, for example, a plasma arc generated from the plasma torch, a method of starting cutting from an end of the steel sheet, and forming a through hole at a predetermined position on the steel sheet to cut from the through hole. There is a way to initiate. The latter is called a piercing start, and according to this piercing start, since the desired shape can be cut and cut in the steel plate surface, the piercing start is usually the mainstream in automatic cutting using NC.

By the way, when the steel sheet is cut by the piercing start, the metal melted by the plasma arc is sputtered (spray of molten metal) between operations called piercing which forms a through hole at the cut start portion by the plasma arc. It blows up, and there is a possibility that this blown up spatter may adhere to the nozzle. Spatters attached to the nozzles cause damage to the nozzles due to melt damage of the nozzles and the occurrence of double arcs, and thus, the sputters are remarkably deteriorated in cutting quality.

Conventionally, in order to avoid damage of the nozzle by such a spatter, as shown in (1) to (4) of FIG. 5 (a), the plasma torch 51 has the highest height that the plasma arc can ignite with respect to the steel plate W. Move to position (h ₁ ) (①), pierce at the height position (② ~ ③), and after the piercing is completed and there is no blow up of the spatter, plasma torch to height position (h ₂ ) suitable for cutting The piercing method of (4) is generally performed by lowering 51 to start cutting. Moreover, the improvement of this conventional generally piercing method is proposed by Unexamined-Japanese-Patent No. 2000-351076. In the piercing method (hereinafter referred to as "rising piercing method") proposed in Japanese Patent Application Laid-Open No. 2000-351076, the plasma torch 51 is shown in Figs. The plasma arc is moved to the upper limit height position (H ₁ ) where the plasma arc can be ignited with respect to the steel plate (W) to ignite the plasma arc at this height position (① to ②). In order to avoid the nozzle damage caused by the plasma arc, the piercing is performed at the height position H ₂ where the plasma torch 51 is raised by a predetermined distance (③), and when the piercing is completed, the height position suitable for cutting ( The cutting is started by lowering the plasma torch with H ₃ ) (4).

Here, if the height for initially forming the plasma arc is defined as "initial height", the piercing height is "pierce height", and the cutting height is defined in the form of "cutting height", respectively, FIG. In the general piercing method shown in a), the initial height h ₁ and the pierce height are set to the same height, and the initial height h ₁ is set higher than the cutting height h ₂ . In addition, in the rising piercing method shown in FIG. 5 (b), the initial height H ₁ is set lower than the pierce height H ₂ , and the initial height H ₁ is higher than the cutting height H ₃ . It is set.

By the way, damage to the nozzle is caused not only by the attachment spatter but also by the pilot arc. That is, since the pilot arc has a high density of thermal energy like the main arc, the longer the time the pilot arc is generated, the higher the degree of melt damage of the nozzle. A technique capable of preventing such a problem is proposed in Japanese Patent Application No. 2002-021284, which is the source of the applicant. In the technique related to the source invention, the switching of the pilot current circuit when forming the pilot arc and the main current circuit when forming the main arc is transistorized, and an appropriate resistance is inserted into the pilot current circuit to insert the pilot arc into the main arc. Is performed at a higher speed and the nozzle is prevented from being melted and damaged by the pilot arc.

However, in the general piercing method and the rising piercing method, damage to the nozzle by the spatter generated during piercing can be avoided, but there is a problem that the nozzle is deteriorated by the pilot arc. That is, in these piercing methods, since the initial height is set at a relatively high position (about twice the cutting height), (A) a larger pilot current is required when growing the pilot arc and shifting to the main arc. ) Since the electrical resistance of the discharge path between the electrode and the steel sheet is extremely large as compared to the electrical resistance of the discharge path between the electrode and the nozzle, the current flows easily into the nozzle, so that the transition from the pilot arc to the main arc is prevented. There is a tendency to be delayed. In these cases (A) and (B), the nozzle is melted excessively by the pilot arc. In addition, there is also a problem that there is a risk of inferior arc ignition by setting the initial height in this way. That is, in the case of (A), it is necessary to set the pilot current high, but when the adjustment of the pilot current is insufficient and the current is low, the force of the pilot arc is weakened, and the arc ignition fails because it cannot be transferred to the main arc. Here, it is also considered to fix the pilot current to the highest value that the instrument can output in order to reduce the effort of adjusting the pilot current, but this results in further accelerating the melt damage of the nozzle by the pilot arc.

In addition, in the technique of the above-mentioned source invention, the effect of suppressing nozzle deterioration to some extent can be expected by the high-speed migration from the pilot arc to the main arc, but the initial height is still set at a relatively high position (about twice the cutting height). Say there is room for improvement.

SUMMARY OF THE INVENTION The present invention has been made in view of the above-described problems and situations, and the nozzle life due to spatter generated at the time of piercing is avoided and the deterioration of the nozzle due to the pilot arc is reliably suppressed, thereby greatly reducing the life of the nozzle. It is an object of the present invention to provide a plasma processing method that can be extended.

Plasma processing method according to the present invention to achieve the above object,

In the plasma processing method of piercing a workpiece with a plasma arc, and starting cutting from the position where the piercing is performed,

(a) a plasma torch having an electrode for forming a plasma arc between the workpiece and a nozzle for ejecting a plasma gas which is a parent of the plasma arc, is moved relative to the piercing position of the workpiece, and the plasma torch First step of relative positioning at an initial height that is equal to, or approximately equal to, the cutting height that is the distance between the plasma torch and the workpiece when cutting the workpiece, and that is a height where no double arc occurs in the vicinity of the workpiece. ;

(b) After positioning of the plasma torch, a pilot arc generated between the electrode and the nozzle is transferred to the main arc between the electrode and the workpiece to form a plasma arc, while maintaining the plasma arc. A second step of relatively moving the plasma torch to a pierce height, which is a position away from the workpiece, than the initial height;

(c) thereafter, a third step of holding the plasma arc with the plasma torch stopped at the pierce height until piercing is complete; And

(d) After the piercing is completed, a fourth step of starting the cutting by relatively moving the plasma torch to the cutting height.                     

According to the present invention, the initial height, which is the distance between the workpiece and the plasma torch when forming a plasma arc between the workpiece and the electrode immediately before piercing, is the cutting height, which is the distance between the plasma torch and the workpiece. It is set to the same or approximately the same height, or to a height where no double arc occurs near the workpiece, so that the pilot arc can be transferred to the main arc with a smaller pilot current than in the prior art, and the discharge path between the electrode and the nozzle Since the difference in the electrical resistance of the discharge path between the electrical resistance and the electrode-machined workpiece decreases, the transition from the pilot arc to the main arc can be performed smoothly, thereby reducing the degree of melt damage caused by the pilot arc. Can be restrained and its life can be extended. It will reap the effects. Further, after forming a plasma arc between the electrode and the workpiece at the initial height, the plasma torch is moved relative to the pierce height, which is a position away from the workpiece than the initial height, while maintaining the plasma arc, and then the piercing is completed. Since the plasma arc is held in the state where the plasma torch is stopped at the height of the pierce, the spatter (spray of molten metal) generated during piercing can be prevented from adhering to the nozzle, and the nozzle by the spatter Damage can be avoided.

In the present invention, the pilot current is interrupted by a semiconductor switch embedded in a line connected to the nozzle in a pilot current circuit for supplying a pilot current to the pilot arc immediately after the transition from the pilot arc to the main arc. It is desirable to. This makes it possible to shorten the time that the pilot current flows between the electrode and the nozzle after the transition from the pilot arc to the main arc, thereby further suppressing the deterioration of the nozzle by the pilot arc.

Next, the specific embodiment of the plasma processing method by this invention is described, referring drawings.

Fig. 1 shows an overall perspective view of the plasma cutting device according to one embodiment of the present invention. 2 (a) is a schematic structural diagram of the main portion of the plasma cutting device in the present embodiment, and FIGS. 2 (b) to 2 (f) show a plasma arc generating circuit and a diagram for explaining the operation thereof. have.

In the plasma cutting device 1 of this embodiment, as shown in FIG. 1, the cutting surface plate (cutting stand) 2 which supports the steel plate W which is a to-be-cut material (processed workpiece) is a rectangular frame ( In addition to being arranged in the inner space of 3), a door-shaped traveling beam 4 is arranged to span the frame 3, and a carriage 5 is arranged on the traveling beam 4, so that the plasma on the carriage 5 is disposed. The torch 6 is mounted.

The traveling beam 4 is capable of traveling in the X-axis direction along the X-axis rail 8 arranged in the longitudinal direction (X-axis direction) of the frame 3 by the drive of the X-axis motor 7, The carriage 5 is capable of traveling in the Y-axis direction along the Y-axis rail 10 disposed on the travel beam 4 by the drive of the Y-axis motor 9. In addition, the plasma torch 6 is movable in the vertical direction (Z-axis direction) with respect to the carriage 5 by the drive of the Z-axis motor 11. In this way, by controlling the respective motors 7, 9 and 11, the plasma torch 6 is moved to an arbitrary position of the steel plate W and positioned at an arbitrary height position so that the cutting processing of the steel plate W is performed. .

The plasma torch 6, as shown in Fig. 2 (a), is a substantially cylindrical shape having a thin tip, and is connected to the plasma power supply unit via the torch cable 12, the relay box 13, and the power cable 14. Steel plate connected to one terminal (negative terminal) of (15), and the other terminal (plus terminal) of the said plasma power supply unit 15 is arrange | positioned in the direction of the plasma torch 6 via the base material cable 16 ( Is connected to W).

Here, the plasma torch 6 is arranged so as to cover the substantially cylindrical electrode 17 arranged at the center position at the tip portion and the outer circumferential side of the electrode 17, as shown in Fig. 2 (b). And a nozzle (18) having a nozzle orifice (18a) having a different diameter on a surface (tip surface) facing the steel plate (W), which is roughly cylindrical, and comprises an electrode (17) and a nozzle (18). There is a working gas passage (R) between. In this working gas passage R, working gas (plasma gas: oxygen in this embodiment) is supplied from a working gas supply system (not shown) from the base end side of the nozzle 18 and the steel plate W from the nozzle orifice 18a. It is supposed to erupt toward you.

The electrode 17 in the plasma torch 6 is connected to the negative terminal of the constant current power supply 15a in the plasma electrode unit 15 via the power supply line 20. In addition, the power supply line from the positive terminal of the constant current power supply 15a includes two pilot current circuits 21 for supplying a pilot current Ip and two main current circuits 22 for supplying a main current Im. Branched to a system line, each of these system lines is connected to the nozzle 18 and the steel plate W, respectively.                     

The high frequency generator 23 is inserted into the power supply line 20, and spark discharge is generated between the electrode 17 and the nozzle 18 by the operation of the high frequency generator 23. 2B is a capacitor for high frequency bypass.

The pilot current circuit 21 has a main arc formed between the electrode 17 and the steel plate W from a pilot arc PA (see FIG. 2 (d)) formed between the electrode 17 and the nozzle 18. MA) The pilot current Ip which flows between the resistor 25, the switching transistor (semiconductor switch) 26, the electrode 17, and the nozzle 18 for smooth transition to [refer FIG. 2 (e)]. The pilot current detector 27 for detecting) is connected in series. Here, for example, when the plasma cutter 1 has a rated pilot current value of about 20 amperes, when the resistance value of the resistor 25 is less than 2 Ω, the inflow current into the nozzle 18 tends to be extremely increased. It is preferable that a resistance value is 2 or more (more preferably 4-8 ohms). In the present embodiment, the control command signal from the controller (not shown) is input to the base of the transistor 26. In addition, the transistor 26 employs a high speed operation such as IGBT as a switching element. In addition, a surge absorption circuit (not shown) composed of a diode or the like may be added to the pilot current circuit 21 to absorb surges during switching as necessary.

The main current circuit 22 has a main current between the electrode 17 and the steel plate W when the main arc MA is formed by a diagram of the pilot arc PA (see FIG. 2 (d) (e)). The main current detector 28 for detecting that (Im) has flown is inserted. In the present embodiment, for example, when a small current of about 3 amps flows through the main current circuit 22, this is detected by the main current detector 28, and the transistor 26 in the pilot current circuit 21 immediately. ), The pilot current Ip flowing between the electrode 17 and the nozzle 18 can be cut off in an instant. In this way, the time which pilot current Ip flows between the electrode 17 and the nozzle 18 is made very short, and the deterioration of the nozzle by a pilot arc is suppressed.

In the plasma arc generating circuit configured as described above, when the start signal is input to the plasma cutting machine 1, the constant current power supply 15a is operated, and as shown in FIG. DC voltage is applied so that the electrode 17 becomes negative and the nozzle 18 and the steel plate W become positive. At the same time, oxygen gas as a pre-flow is supplied to the working gas passage R in the plasma torch 6. In addition, the preflow is performed in order to completely replace the air in the working gas passage R with oxygen and to obtain a time allowance until the gas flow rate is stabilized. After the pre-flow, as shown in FIG. 2C, when a high frequency high voltage is applied between the electrode 17 and the nozzle 18 by the operation of the high frequency generator 23, the electrode 17 and the nozzle 18. A spark discharge occurs in between, and as a result of the spark discharge, a pilot arc PA is formed between the electrode 17 and the nozzle 18, as shown in FIG. 2 (d), from the constant current power supply 15a. The pilot current Ip flows through the resistor 25 and the transistor 26 to the circuit which returns from the nozzle 18 and the pilot arc PA to the constant current power supply 15a via the electrode 17. At this time, since the constant current power supply 15a is in a state showing the maximum output, that is, functioning as a substantially constant voltage source, the pilot current Ip is given a drop characteristic by the resistor 25, It is stable when the power supply characteristics and arc voltage are balanced.

Subsequently, as shown in FIG. 2E, when the electrical conduction is secured between the electrode 17 and the steel plate W with the pilot arc PA as a lead, a part of the pilot current Ip becomes the main current ( Im) and flows into the steel plate W to form the main arc MA. This is detected by the main current detector 28 and the pilot current Ip flowing between the electrode 17 and the nozzle 18 by the off action of the transistor 26 as shown in Fig. 2 (f). It becomes the circuit only of the main arc MA (plasma arc), and only the main current Im flows. Further, constant current control is performed while comparing the output value of the main current detector 28 with the set value so as to maintain the preset cutting current value (main current Im), and piercing or cutting is performed on the steel plate W. .

Subsequently, a piercing for forming a through hole in the cut start portion of the steel sheet W by the plasma arc generated in this way will be described in sequence for each step with reference to the piercing operation explanatory diagram in FIG. 3. In this case, the piercing in the case of cutting a mild steel sheet having a plate thickness of 19 mm by piercing start with an oxygen plasma cutting machine having an output of 90 A (amps) will be described as an example.

[Step 1: See FIG. 3 (a)]

The center of the plasma torch 6 is moved while performing free flow to coincide with the cutting start position (piercing position) previously designated by the NC device (not shown), and the center of the plasma torch 6 coincides with the piercing position. Plasma torch 6 and steel plate W at the time of cutting the initial height FH, that is, the distance between the plasma torch 6 and the steel plate W when generating a plasma arc to start piercing. Positioning is made equal to 3 mm equal to the cutting height CH, which is the distance of (step 1).

[Step 2: See FIG. 3 (b) (c)]

After completion of the positioning of the plasma torch 6 in the step 1, a plasma arc is generated between the electrode 17 of the plasma torch 6 and the steel plate W, and thereafter, a spatter (spray of molten metal) In order to avoid the spatter adhesion to the nozzle 18 by the blow-up of the pierce height, the pierce height which is the height which can avoid the spatter at the position which is further from the steel plate W rather than the initial height FH, holding the said plasma arc. The plasma torch 6 is raised to (PH) (PH = 15 mm) at 100 mm / min faster than the speed at which the piercing proceeds. In addition, the pilot current Ip at the time of generating a plasma arc here is 9A from the correlation of pilot current-initial height of FIG. 4 (a).

[Step 3: See FIG. 3 (c)]

After the step 2, the plasma arc is held while the plasma torch 6 is stopped at the pierce height PH until the piercing is completed. At this time, the holding time is about 1 second.

[Step 4: See FIG. 3 (d)]

After completion of the piercing according to the above step 3, the plasma torch 6 is lowered to the cutting height CH (CH = 3 mm), which is an optimum height at which a good cutting surface can be obtained, and cutting is started.

In the piercing of the present embodiment, since the initial height FH is 3 mm, the pilot current Ip is 9 A from Fig. 4 (a), and the nozzle life frequency [the predetermined level of cutting quality can be maintained from Fig. 4 (b). Number of firings] was about 1000 times. Conventionally, FH = 8mm with respect to CH = 3mm, Ip = 24A from FIG.4 (a), and therefore nozzle life frequency was about 300 times from FIG.4 (b). In other words, the nozzle life was extended by more than twice as compared with the prior art.

According to the present embodiment, damage to the nozzle 18 which greatly affects the cutting quality can be greatly reduced not only by the spatter but also by the pilot arc PA, so that the nozzle life can be extended. Not only can the process cost associated with the replacement of the nozzle 18 be reduced, but also the cutting quality can be stably maintained for a long time, resulting in a significant improvement in workability.

Moreover, according to this embodiment, since the initial height can be set to a low position without depending on the plate thickness, there is an advantage that the transition from the pilot arc PA to the main arc becomes easy, and the ignition reliability is improved. In addition, since the initial height can be made constant without depending on the plate thickness, there is an advantage that the adjustment of the pilot current Ip is unnecessary and the handling becomes easy.

Claims (2)

In the plasma processing method of piercing a workpiece with a plasma arc, and starting cutting from the position where the piercing is performed, (a) a plasma torch having an electrode for forming a plasma arc between the workpiece and a nozzle for ejecting a plasma gas which is a parent of the plasma arc, is moved relative to the piercing position of the workpiece, and the plasma torch A first step of performing relative positioning at an initial height that is equal to a cutting height that is a distance between the plasma torch and the workpiece when cutting the workpiece, or a height at which a double arc does not occur in the vicinity of the workpiece, than the cutting height; (b) After positioning of the plasma torch, a pilot arc generated between the electrode and the nozzle is transferred to the main arc between the electrode and the workpiece to form a plasma arc, while maintaining the plasma arc. A second step of relatively moving the plasma torch to a pierce height, which is a position away from the workpiece, than the initial height; (c) a third step of holding the plasma arc with the plasma torch stopped at the pierce height until piercing is complete; And (d) Plasma processing method characterized by having a 4th process of starting a cutting | disconnection by relatively moving a plasma torch to the said cutting height after completion of the said piercing. 2. The pilot current according to claim 1, wherein in the pilot current circuit for supplying pilot current to the pilot arc immediately after the transition from the pilot arc to the main arc, the pilot current is inserted into a line connected to the nozzle. Plasma processing method characterized in that the blocking.

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